The present invention relates to a unit for printing a web of paper comprising a frame and at least a first and a second printing group, each printing group comprising a blanket cylinder and a plate cylinder, the printing unit also comprising a mechanism for supporting and moving the cylinders, by means of which mechanism the printing unit has at least one throw-on configuration, in which the blanket cylinders are pressed against each other and against the plate cylinders, and a throw-off configuration, in which the blanket cylinders are spaced apart from each other and from the plate cylinders.
The invention is used in particular for offset presses, for example, for printing books.
The throw-off configuration allows, for example, the blankets of the blanket cylinders to be changed.
The throw-on configuration allows the printing unit to print the web of paper which passes between the blanket cylinders.
In addition to these two configurations, the unit generally has an intermediate configuration between the throw-off and throw-on configurations. In this configuration, which in particular allows plates to be changed, the blanket cylinder of each printing group is pressed against the plate cylinder of the same group but remains spaced apart from the blanket cylinder of the other printing group.
The throw-off configuration and the plate-changing configuration allow the web of paper to pass between the blanket cylinders of the two printing assemblies and thus to pass through the printing unit which is in the idle state. The web of paper can at the same time continue to be printed by other printing units.
This allows a printing operation to be carried out by some units within the same printing press whilst other units in the idle state are prepared for the following printing operation.
A press of this type is generally referred to as an “Auto Transfer” press.
FR-2-787 059 describes a printing unit of the above-mentioned type wherein the web of paper moves horizontally. The first printing group is thus arranged above the web of paper and the second printing group below. The support mechanism comprises arms which are articulated to the frame and on which the cylinders of the two assemblies pivot. The movement from the throw-on configuration to the throw-off configuration is carried out by raising the upper arms and therefore the cylinders of the upper printing group and by lowering the lower arms and therefore the cylinders of the lower printing group. Conversely, the movement from the throw-off configuration to the throw-on configuration is carried out by lowering the upper arms and cylinders and by raising the lower arms and cylinders.
During the lowering movements, the cylinders are driven by their own weight in such a manner that their movements would normally be limited by stops.
Although a kinematic system of this type is acceptable for cylinders having small dimensions which are, for example, intended to print 8 or 12 tabloid pages per printing group, it is unsuitable for cylinders which have larger dimensions and therefore masses and which, for example, allow up to 48 tabloid pages to be printed per printing group.
The lowering movements of the cylinders could bring about significant impacts, in particular on the stops, and lead to occurrences of damage.